LED Mortar Board Battles Suns Brightness With 21W Of Power

[Jeremy Blum] aimed to be the brightest student at his Master’s graduation ceremony this spring. He designed an LED rig for his mortar board which should battle the sun’s intensity by using up to 21 watts of power. But he didn’t stop with eye-catching intensity. while he was at it he also included some interactive features so the guy behind him has a way to keep from going blind.

One thing that really caught our eye is the 3D printed parts he generated for the project. There’s a nice mounting plate for the LED side of things, and a wrist-mounted enclosure for the Raspberry Pi board. Wait, why does he need an RPi to drive some LEDs? We already mention interactivity which is facilitated by the Pi acting as a WiFi hotspot. Connect to the access point and choose a color. If you’re in the seat behind [Jeremy] you’ll want to choose black! All of this and is explained in his video presentation.

The article states, “It is equipped with a total of 16 350mA high-brightness LEDs (4 each of Red, Green, Blue, and White). The PCB is fed 15V and drops it over each of the 4 LED colors in series (4 parallel strings of 4 LEDs).”

No you won’t because the thermal load quadruples with doubling of current. Halving the difference between heatsink temperature and ambient temperature requires doubling the heatsink, or doubling airflow etc. That means the temperature of the heatsink can only get so low even with ridiculously large heatsinks because of diminishing returns.

So the bottleneck is the thermal interface from the LED junction to the heatsink, and doubling of current and quadrupling the heat load will cause a near-fourfold increase in junction temperature over the ambient, and the LED will overheat and dim within seconds. Permanent damage occurs within minutes. That’s why you don’t run LEDs at double their rated current.

The forward voltage of the LED junction increases a lot with increasing current and is highly variable depending on the exact part, which means your power dissapation is not as linearily related to the current as it tends to be in small signal diodes: http://files.leds.co.uk/images/news/archive/figure1.gif

It’s not quite a power of two relationship, but the higher temperature also increases the ohmic resistance in other parts of the chip, like the bonding wires, which also leads to increased heat dissapation inside the chip.

Not sure why he used a usb-ftdi cable, he says the light control was done by an arduino clone, which I’m assuming to be a bootloadered atmega, so why didn’t he just use uart on the gpio? There’s no need for it to be particularly quick, 9600 should be plenty for a few bytes, so it could have been bit banged if necessary.

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